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The
following table displays the spot size in microns for different
f-ratios at various frequencies of light. The spot size is the diameter
of the Airy disc. It indicates the smallest size a point source such as
a star image will produce on your image detector (film, CCD, etc.). You
can use this to work out if your imaging system is optimized for
resolution. Of course you will also need to know the film grain or
pixel size on your imaging detector.
The formula used is (spot size) = 2.44 * (wave length of light) * (f-ratio of optical system).
Please note that the above is the Rayleigh criterion and is
theoretical. In reality, the best resolution achievable by your imaging
system depends on both its quality as well as items that could be
beyond your ability to control such as atmospheric conditions. The
Rayleigh criterion is empirically derived and indicates the optical
resolution acieivable with the human eye. The spot sizes indicated in
the table are twice the Rayleigh criterion.
Spot size in microns for different f/ratio optical systems at different wavelengths of light.
| f/ratio |
300 nm |
400 nm |
500 nm |
550 nm |
600 nm |
650 nm |
700 nm |
800 nm |
900 nm |
1000 nm |
| f/1 |
0.73 |
0.98 |
1.22 |
1.34 |
1.46 |
1.59 |
1.71 |
1.95 |
2.20 |
2.44 |
| f/2 |
1.46 |
1.95 |
2.44 |
2.68 |
2.93 |
3.17 |
3.42 |
3.90 |
4.39 |
4.88 |
| f/3 |
2.20 |
2.93 |
3.66 |
4.03 |
4.39 |
4.76 |
5.12 |
5.86 |
6.59 |
7.32 |
| f/4 |
2.93 |
3.90 |
4.88 |
5.37 |
5.86 |
6.34 |
6.83 |
7.81 |
8.78 |
9.76 |
| f/5 |
3.66 |
4.88 |
6.10 |
6.71 |
7.32 |
7.93 |
8.54 |
9.76 |
10.98 |
12.20 |
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| f/6 |
4.39 |
5.86 |
7.32 |
8.05 |
8.78 |
9.52 |
10.25 |
11.71 |
13.18 |
14.64 |
| f/7 |
5.12 |
6.83 |
8.54 |
9.39 |
10.25 |
11.10 |
11.96 |
13.66 |
15.37 |
17.08 |
| f/8 |
5.86 |
7.81 |
9.76 |
10.74 |
11.71 |
12.69 |
13.66 |
15.62 |
17.57 |
19.52 |
| f/9 |
6.59 |
8.78 |
10.98 |
12.08 |
13.18 |
14.27 |
15.37 |
17.57 |
19.76 |
21.96 |
| f/10 |
7.32 |
9.76 |
12.20 |
13.42 |
14.64 |
15.86 |
17.08 |
19.52 |
21.96 |
24.40 |
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| f/11 |
8.05 |
10.74 |
13.42 |
14.76 |
16.10 |
17.45 |
18.79 |
21.47 |
24.16 |
26.84 |
| f/12 |
8.78 |
11.71 |
14.64 |
16.10 |
17.57 |
19.03 |
20.50 |
23.42 |
26.35 |
29.28 |
| f/13 |
9.52 |
12.69 |
15.86 |
17.45 |
19.03 |
20.62 |
22.20 |
25.38 |
28.55 |
31.72 |
| f/14 |
10.25 |
13.66 |
17.08 |
18.79 |
20.50 |
22.20 |
23.91 |
27.33 |
30.74 |
34.16 |
| f/15 |
10.98 |
14.64 |
18.30 |
20.13 |
21.96 |
23.79 |
25.62 |
29.28 |
32.94 |
36.60 |
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| f/16 |
11.71 |
15.62 |
19.52 |
21.47 |
23.42 |
25.38 |
27.33 |
31.23 |
35.14 |
39.04 |
| f/17 |
12.44 |
16.59 |
20.74 |
22.81 |
24.89 |
26.96 |
29.04 |
33.18 |
37.33 |
41.48 |
| f/18 |
13.18 |
17.57 |
21.96 |
24.16 |
26.35 |
28.55 |
30.74 |
35.14 |
39.53 |
43.92 |
| f/19 |
13.91 |
18.54 |
23.18 |
25.50 |
27.82 |
30.13 |
32.45 |
37.09 |
41.72 |
46.36 |
| f/20 |
14.64 |
19.52 |
24.40 |
26.84 |
29.28 |
31.72 |
34.16 |
39.04 |
43.92 |
48.80 |
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| f/21 |
15.37 |
20.50 |
25.62 |
28.18 |
30.74 |
33.31 |
35.87 |
40.99 |
46.12 |
51.24 |
| f/22 |
16.10 |
21.47 |
26.84 |
29.52 |
32.21 |
34.89 |
37.58 |
42.94 |
48.31 |
53.68 |
| f/23 |
16.84 |
22.45 |
28.06 |
30.87 |
33.67 |
36.48 |
39.28 |
44.90 |
50.51 |
56.12 |
| f/24 |
17.57 |
23.42 |
29.28 |
32.21 |
35.14 |
38.06 |
40.99 |
46.85 |
52.70 |
58.56 |
| f/25 |
18.30 |
24.40 |
30.50 |
33.55 |
36.60 |
39.65 |
42.70 |
48.80 |
54.90 |
61.00 |
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